This invention relates to a mixed roll calender arrangement having a combination of hard and soft rolls which improves both the smoothness and gloss uniformity of a paper web and particularly a coated paper web.
The calendering of a paper web leaving the discharge end of a paper machine is a well known final finishing treatment for determining the smoothness and gloss of the surfaces of the paper as well as its consistency. Such calendering is generally accomplished by guiding a continuous paper web successively through a series of nips formed by calendering rolls.
Conventionally, a paper web is calendered in a stack of rolls which may be a machine finishing stack in which the rolls are all iron or steel rolls and the nips defined by such rolls are called hard nips. Such machine calendering treatment may be complemented, if necessary, by a supercalendering or soft calendering treatment in a separate supercalender. In the supercalender arrangement, the rolls are commonly covered with a resilient material, and these covered rolls are used in combination with iron or steel rolls to form soft nips.
It is generally accepted that the influence of calendering on a web to be processed is the production of higher gloss on the surface facing the hard roll in the nip. There are, however, several factors which affect the gloss and smoothness of the web such as nip load, speed, temperature, sheet moisture, and the material used to form the elastic covering.